Some embodiments provide a method for a first device to synchronize a set of data items with a second device. The method receives a request to synchronize the set of data items stored on the first device with the second device. The method determines a subset of the synchronization data items stored on the first device that belong to at least one synchronization sub-group in which the second device participates. Participation in at least one of the synchronization sub-groups is defined based on membership in at least one verification sub-group. The first and second devices are part of a set of related devices with several different verification sub-groups. The method sends only the subset of the synchronization data items that belong to at least one synchronization sub-group in which the second device participates to the second device using a secure channel.